Avoiding Turbulent Times
While some landings may be mandatory, every takeoff is optional
By Valerie Salven

I WAS TURNING DOWNWIND FOR MY SECOND TOUCH-AND-GO LANDING when I heard the tower clear a C-130 "for the option." As I wondered what kind of "options" a bulky, lumbering military transport such as the C-130 could possibly have, the tower cleared me with a "Number two for a touch-and-go behind the C-130." The controller added, "Caution: wake turbulence."

"Trailing vortices" have a sink rate of several hundred feet/minute, depending on the aircraft. Wake turbulence abates only when enough time has pased or there's enough distance between the originting aircraft and the plane behind it.

I turned base as the C-130 passed me, heading in the opposite direction on its final approach. As I turned final, I watched the C-130 touch down 1000 feet beyond the threshold of the 7000-foot runway. I adjusted my descent so that I'd touch down about 200 to 300 feet beyond his touchdown point.

Then, I scanned the length of the runway and noted with surprise that the C-130 was airborne again and climbing briskly on the runway heading. It didn't look so lumbering anymore. My impression was that the climb performance of the Cessna 172 that I was in wasn't nearly enough to keep me out of the wake of the C-130 if I attempted the "go" part of my "touch-and-go." I keyed the mic and informed the tower that I wanted to make a full-stop landing.

When I returned home, I decided it was a good time to review wake turbulence procedures. I was also curious about the kinds of wake turbulence encounters that have been reported to the NTSB.

What The AIM Says About Wake Turbulence
Section three of the Aeronautical Information Manual (AIM) discusses wake turbulence in some detail. It notes that every aircraft generates a wake while in flight. The basic factor in "vortex generation"—the creation of a wake—is weight, and the greatest vortex strength occurs when the generating aircraft is "heavy, clean and slow."

The responsibility of VFR pilots for the avoidance of wake turbulence encounters is so important that the FAA spells it out in capital letters at subsection 7-3-6:

WHETHER OR NOT A
WARNING HAS BEEN GIVEN,
PILOTS ARE EXPECTED TO
ADJUST THEIR OPERATIONS
AND FLIGHTPATH(S) AS
NECESSARY TO PRECLUDE
SERIOUS WAKE ENCOUNTERS.

In other words, don't rely on ATC to assess all possible hazards of a situation for you. ATC is required to provide separation of IFR aircraft for wake turbulence purposes, but the pilot in command of a VFR aircraft (and, presumably, pilots of IFR aircraft operating in visual meteorological conditions) must be observant of the activity of nearby "large" (over 41,000 pounds) or "heavy" (over 255,000 pounds possible takeoff weight) aircraft so that precautionary measures may be taken, if necessary, to prevent a possible wake encounter.

The FAA's Guide To Avoiding Wake Turbulence

1 Takeoff. If you think that wake turbulence from the preceding aircraft may be a factor, tell the tower you'd like to wait—before taking the runway. Plan your takeoff, so you'll lift off before the rotation point of the other aircraft.

2 Climb. If possible, climb above the flightpath of the preceding aircraft. When that is not possible, deviate slightly upwind. Avoid headings that cause you to cross behind and below the preceding aircraft in order to avoid turbulence.

3 Crossing. If you must cross behind the preceding aircraft, try to cross above its flightpath or (terrain permitting) at least 1000 feet below.

4 Following. Stay either on or above the flightpath of the preceding aircraft.

5 Approach. Maintain a position on or above the preceding aircraft's flightpath.

6 Landing. Ensure that your touchdown point is beyond the preceding aircraft's touchdown point or land well before the departing aircraft's rotation point.

7 Crossing Approaches. When landing behind another aircraft on crossing approaches, it's wise to cross above the other aircraft's flightpath.

8 Crosswinds. Remember that crosswinds may affect the position of vortices. Adjust takeoff and landing points accordingly.

9 Helicopters. Helicopter wake vortices may be of significantly greater strength than fixed-wing aircraft of the same weight. Avoid flying beneath the flightpath of helicopters.
The "trailing vortices" that constitute wake turbulence from heavy aircraft have a sink rate of several hundred feet per minute. These vortices diminish in strength with the passage of time and distance behind the generating aircraft. Wind and weather conditions also play a role in the dissipation of wake turbulence. While we've all received training about the behavior of these vortices, a look at NTSB reports illustrates risks in situations that you might not have thought of before.

The pilot of a Piper PA-28-140 became lost on a cross-country flight over North Carolina. He decided to make a precautionary landing on an interstate highway as his fuel supply neared exhaustion. He touched down about 300 feet behind a tractor-trailer truck and lost control of the aircraft during the landing roll. The aircraft veered off the right side of the highway and nosed over. This pilot concluded that he lost control of his aircraft as a result of "wake turbulence" from the truck.

Less unusual are reports of wake turbulence encounters involving wake generated by what the NTSB refers to as the "accident aircraft." The pilot of a Cessna 152F, for example, was seriously injured when he lost control of his aircraft after flying into his own wake turbulence as he orbited 800 feet above a school of fish off the Atlantic coast.

The commercial pilot of a Bell 47-GB31 helicopter flew into his own wake as he made a fourth pass over an area to dispense a "rinse load" in Colorado. The helicopter settled onto the ground in a nose-low attitude that caused the main rotor blades to strike the tail boom and sever it from the aircraft.

Helicopter wake turbulence has created problems for fixed-wing aircraft, too. There's a report in the NTSB files of a Cessna 172G that flipped in a nearly inverted position and impacted the ground 500 feet short of a runway in Mississippi when its pilot attempted to follow 1/4-mile behind a UH-1 helicopter landing at an uncontrolled airport. The Cessna pilot stated afterward that he wasn't aware that helicopters produced wake turbulence.

A Bonanza pilot learned the hard way that he had followed an Army AH-1S helicopter too closely on final approach at Delaware, Ohio. The Bonanza crossed the runway threshold while the helicopter was still about 250 feet from a planned touchdown point of 1500 feet down the runway. The Bonanza rolled left, struck the ground with its left wing, continued across a taxiway and came to rest in an aircraft parking area.

A commercial pilot with more than 2000 hours of flight time had been cleared by the tower of an airport in Texas to taxi a twin-engine Cessna 310J into position and hold while an EMB-145 regional jet (a Brasilia) made a low pass over the airport. After the Cessna pilot reported the jet in sight, the tower cleared the Cessna for takeoff.

"The pilot immediately commenced takeoff as the jet passed over," the NTSB report states. "No cautions for wake turbulence were issued by tower personnel, nor did the pilot request a delay for takeoff."

The Cessna rotated at 100 knots. After a positive rate of climb was achieved, the landing gear was retracted. The Cessna immediately began to roll to the right, the right wing struck the ground and the aircraft caught fire. Its pilot sustained serious injuries.

Wake turbulence will always be a part of aviation, and avoiding it will always be on the long list of a pilot's responsibilities.